Not applicable.
The invention relates to a method and a system for controlling a headbox in a paper machine or in a board machine.
The profile faults which occur in fibre orientation on paper/board machines have been substantially reduced by means of dilution technology, but unevenness of fibre orientation can still be observed in paper, which unevenness appears as a so-called S-profile (the curve plotted with an unbroken line in FIG. 4). The S-profile in fibre orientation may be caused, for example, by an uneven pressure profile of a stock inlet header or a dilution inlet header, which gives rise to unevenness in flow rate profiles. The regulated consistency profile may also produce an undesirable pressure loss coefficient profile across a turbulence generator. The effect of this on the flow rate profile is seen such that at those points where the pressure loss coefficient is small, the flow rate increases and, in a corresponding manner, at points of high pressure loss coefficients, the flow rate decreases.
Another quantity difficult to control is the control of the jet speed in particular in headboxes with trailing elements. The problem is encountered both when using turbulence trailing elements and in multi-layer headboxes. In accordance with the state of the art, the jet velocity is predicted by means of a pressure measured from a side wall of a slice channel. However, this measurement method is inaccurate, for example, because of the flow disturbances arising from additional feeds and from trailing elements. Moreover, if there is an uneven pressure profile in the width direction of the slice channel, a value that has been measured from the side wall does not provide any information about pressure values elsewhere in the slice channel or in the cross direction of the machine.
An object of the invention is to develop a method and a device for measuring the flow rate profile in a slice channel of a headbox. Additionally, an object of the invention is to develop a control method for controlling the fibre orientation profile based on the flow rate profile. The flow rate profile can be measured either directly or indirectly.
The method for controlling a headbox of a paper/board machine according to the invention is mainly characterized in that the headbox is provided with cross direction measuring sensors, by means of which the flow rate profile in the slice channel of the headbox is determined, and the profile bar of the slice opening of the headbox is adjusted in the cross direction on the basis of the thus determined flow rate profile.
The measurement and control system according to the invention is in turn characterized in that the headbox is provided with measuring sensors for determining the cross direction flow rate profile in the slice channel of the headbox and with means for adjusting the profile bar on the basis of the flow rate profile.
The method and the system according to the invention allow the flow rate profile to be measured from the slice channel in the cross direction and/or in the machine direction. Based on accurate determination of the flow rate profile, the profile bar is adjusted such that cross velocities and orientation angles are minimised. In the measurement of the flow rate profile it is possible to use several methods, which are described further on. The invention provides correction of fibre orientation profiles which is more accurate than before.
In the following, the invention will be described with reference to the graphic representations shown in the accompanying figures and illustrating the invention and to a drawing of principle showing a measurement system of a headbox according to the invention, to which the invention is not intended to be exclusively confined.